In a conventional heating furnace to which this invention relates, a gas atmosphere which works to prevent articles under heat treatment from oxidation and decarbonization, is discharged gradually from the furnace to the air, and replenished regularly by a fresh gas. Such fresh gas atmosphere is commonly made in a gas generating unit which is independent from the heating furnace and employs mostly a catalytic metal retort, and is supplied into the furnace after having been cooled. This is really a waste of gas atmosphere.